Tetrahedral twist

Racemic amino acids have been resolved via stereoselective coordination to the square planar chiral matrix complex (178).584 The bisamidobispyridyl ligand (179) forms a square planar Ni11 complex with considerable tetrahedral twist due to repulsion of the ortho protons of the pyridyl rings.585... 

The (bme-daco)Ni11 complex is a purple solid that is soluble in a variety of solvents, including methanol, acetonitrile, water, and acetone. The UV-vis in MeCN displays a broad band at 506 nm (640) with a shoulder at 602 nm. The HNMR spectrum is complex due to a tetrahedral twist in the N2S2 plane. Mass spectrum m/z = 291. 

Fig.31. Idealized subdivision of the dioctahedral 2 1 clay minerals Numbers refer to amount of calculated tetrahedral twist. = no octahedral Fe = high octahedral Fe3+ = high octahedral Al.

In the dioctahedral 2 1 sheet-structure silicate with the occupied sites more than 85% occupied by Al, the structure seems to be able to compensate for the internal strain and can grow to a considerable size. The Al octahedral occupancy values of muscovite (>1.7) and the 2 1 dioctahedral clays (1.3—1.7) indicate that there is little overlap. It is likely that the decreased amount of tetrahedral twist induced by increasing the size of the octahedral cations and octahedral charge (decreasing Al) determines that a clay-size rather than a larger mineral will form. The R3+ occupancy value can be less than 1.3 when the larger Fe3+ is substituted for Al. When Al occupancy values are less than 1.3 (65%), in the absence of appreciable iron, the internal strain is such that growth is in only one direction. The width of the layer is restricted to five octahedral sites. Sufficient layer strain accumulates within this five-site interval such that the silica tetrahedral sheet is forced to invert to accommodate the strain. 

Force Field Parameters V Parameterizing a New Potential -The Tetrahedral Twist of Four-Coordinate Compounds. 257... 

Figure 10.3 Correlations between tetrahedral twist angle 6 and properties of tetraaminecopper(II) complexes...

The Structure module of MOMEC enables you to analyze structures that have been saved as. hin files. These can be structural data files from experimental work, from a data base (e.g. the CSD) or computed structures such as those optimized with MOMEC. The geometric parameters accessible include the calculation of a least-squares plane (defined by three or more points), the distance of atoms from this plane, the angle between a vector such as a metal-ligand bond and a plane, that between two planes, e. g., for the measure of a trigonal twist angle or a tetrahedral twist angle. In this lesson, we will analyze the structures of the four conformers of [Co(en)3]3+ considered in Sections 17.3, 17.4 and 17.5. 

An important geometric parameter for four-coordinate systems is the tetrahedral twist angle 9 (Fig. 17.7.3). We can use this parameter to inspect the geometry around one of the coordinated amine donors. Define a new reference plane with one of the N donors and two of the atoms bound to it (e. g., the Co center and one of the FI s). Then, click on the Angle between two planes button and follow the instruction to define the other plane (the same N donor again and the other two substituents, i. e., the second H and the C atom bound to the N). The value of the tetrahedral distortion (0 = 88.5°) will then be displayed (the value of 9 obviously depends on the choice of planes). 

As one of the special features, MOMEC has a plane twist function. This has been included to limit the tetrahedral twist in four-coordinate compounds, where 1,3-nonbonded interactions lead to a preference for a tetrahedral arrangement (see Section 3.6). That is, the plane twist potential can be used to induce a square-planar arrangement or, using constraints, any intermediate structure can be enforced. The same potential can in principle be used for other structural features (see Fig. 17.14.1), such as the Bailar twist of six-coordinate complexes or for computing the rotational barrier of metallocenes. However, at present it has only been implemented in MOMEC for the tetrahedral twist and no parameters have been included as yet. 

Using out-of-plane potentials. This is probably the most reasonable technique from those available in older programs but it is not very intuitive and it does not allow for the constraint of specific tetrahedral twist angles. 

In a tetrahedral coordination polyhedron there are three possible tetrahedral twist angles y. MOMEC automatically chooses one of them (you may want to have two twist angles included this can be done by changing the default value for twist in the momec.ini file from 1 to 2). 

There is little change in most values, particularly for the 5X isomer which has little tetrahedral twist. The increase in strain energy in the XX isomer is mainly due to a build up of torsional strain, van der Waals repulsion and twist angle strain. Minimization does not proceed as smoothly with larger values of the force constant. Thus, it may be advisable to increase the damping and/or to decrease the termination rms shift (Setup/Optimization Controls). 

Model the other Pt complexes described in Section 17.6 using a tetrahedral twist function rather than the previously used out-of-plane functions. Establish what values of the force constant ky are necessary to enforce planarity in the different complexes. 

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